[amsat-bb] Re: The Moon is our Future

tosca005@umn.edu tosca005 at umn.edu
Thu Jul 2 08:52:11 PDT 2009

On Jul 2 2009, kd8bxp at aol.com wrote:

> Don't want to get a whole new thing started here but - I don't think we 
> ever went to the moon in 1969 and I don't think we will ever goto the 
> moon - in 2012 or whenever they proposed a "return" to the moon

With that level of disbelief I can certainly see why you are predisposed to 
discount the possibility of a moon-based transponder.

> I would love to see an amatuer repeater on the moon thou - from my 
> understanding EME is expensive to do, so I think it would leave most of 
> us out

Not necessarily. Remember, with conventional EME, you send as large a 
signal as you can muster towards the moon, incur huge path losses along the 
way, then incur a huge loss because the moon is a very imperfect reflector 
of RF energy, then incur the huge path loss back from moon to earth.

With a moon-based repeater, you send as large a signal as you can muster 
towards the moon, incur the same path loss from earth to moon; BUT, THEN 
you enlarge the signal with a gain antenna at the repeater, and then have a 
sensitive receiver that can detect and amplify the signal. The repeater 
then transponds the signal to a different frequency band, amplifies it as 
much as equipment weight and power availability allow, transmit it through 
a gain antenna, and only THEN incur the huge path loss from moon to earth.

Because the path loss is only in a single direction, and instead of an 
inefficient (lossy) passive reflector, you have gain antennas for reception 
and transmission, plus amplification on receive and on transmit, the net 
earth station requirements should be much less than conventional EME.

That's not to say it would be easy, just that it should be easier than 
conventional EME in terms of station requirements on earth. As has been 
mentioned numerous times already, the station requirements for the space 
end of the system are enormously more difficult than anything we've ever 
tackled so far with either LEO or HEO satellites.

But there's no harm in DISCUSSING the idea, and learning about the pitfalls 
and possibilities as part of the discussion. Even if it never comes to 
pass, we should all be a bit more knowledgeable after having had the 

As far as the dreaming goes, wouldn't an L/S transponder be better than a 
V/U or U/V transponder? Granted, the path loss is greater, but the antenna 
gain is easier to produce...

While I am a firm believer in the KISS principle (Keep It Simple, Stupid!), 
I am getting a little tired of hearing people complain endlessly about the 
downfall of AO-40 being due to its complexity. Uhh, the downfall of AO-40 
was human error, which will ALWAYS be an issue. The only reason that AO-40 
was ever usable at all was BECAUSE of its complexity, i.e., the redundancy 
of multiple transponders that could be switched into place after initial 
failures, etc. OF COURSE, a mission to the moon needs to be as light and 
compact as it can be made, and therefore much simpler than AO-40, but due 
to the harsh environment in which it would be asked to operate, it needs to 
be as complex as necessary to get the job done, i.e. not as simple as AO-10 
or AO-13.

73 de W0JT

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